Aircraft water drains, aircraft fuel system water drains.

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Airworthiness Articles AAC 1-119 Amdt 1Water Drains in Aircraft Fuel Systems7/2001

Water in the fuel system of an aircraft can cause corrosion of components in the fuel system and interrupt the correct operation of aircraft engines. 

Water can enter directly with fuel, via an unserviceable fuel cap, or breathing of air in the tanks due to changes in outside air temperature or the aircraft ascending and descending.

Aircraft are designed to have this water safely drain from the aircraft.  The current standards are:

FAR 23.971 Fuel tank sump.

(a)   Each fuel tank must have a drainable sump with an effective capacity, in the normal ground and flight attitudes, and

(b)   Each fuel tank must allow drainage of any hazardous quantity of water from any part of the tank to its sump with the aircraft in the normal ground attitude.

FAR 23.999 Fuel System Drains.  There must be at least one drain to allow safe drainage of the entire fuel system with the aircraft in its normal ground attitude.

Previous Australia certification standards, such as CAO 101.22 para 3.2(1), included similar requirements.  All aircraft certificated for operation in Australia are therefore required to have suitable water drain facilities.

Unfortunately, experience during the AVGAS fuel crisis demonstrated that some aircraft do not have satisfactory water drain facilities.  Also, there is evidence that some fuel tanks provided as spares for Tiger Moth aircraft do not have suitable water drain facilities.  Finally, Transport Canada FEED-BACK 3/4/2000 noted corrosion problems they have recorded due to the presence of water in fuel.

Description of Problem

Aircraft may not have suitable water drain facilities.  Water drain samples taken on daily flights may therefore not be representative of fuel in the aircraft, which could result in corrosion of vital fuel system components.

Tiger Moth Aircraft

When the Tiger Moth aircraft is in flight the sump is the lowest point, but when the aircraft is on the ground (tail down) the sump is not the lowest point on the fuel tank. When these aircraft were used for agricultural operations the aircraft were often left standing in all weather and a large amount of water frequently resulted in the tanks.  Many operators fitted two extra drains at the lowest point in the tanks (tail down) to drain excess water that the sump could not handle.

Spare tanks are delivered with only one drain fitted.  Fitment of the extra drains is not mandatory, and may not be necessary if the aircraft is regularly sheltered.  Operators of Tiger Moth aircraft should however consider the need to have extra drain points installed.

Recommended Continuing Maintenance Actions

The fuel system must be inspected at every 100 hour maintenance action.  Should this inspection reveal evidence of water remaining in the fuel system, maintenance staff should test the fuel system for continuing compliance with the basis of certification, as described above.

One way to test the water drain facilities is to add a small measured amount of water into a fuel tank while the aircraft is on a level surface and the aircraft is in the normal ground attitude.  The same amount of water should then move to the tank sump, and be drained from the aircraft.  Any unexpected missing amount of water is evidence that the fuel system does not meet the drain criteria.

FAA AC 20-125, Water In Aviation Fuels, para 7 c suggests that it may be necessary to gently rock the wings of some aircraft while draining the sump in order to completely drain all the water.

The water added must be clean; normal drinking water is adequate.  The amount of water added should be appropriate to the tank size, and not exceed the minimum sump capacity, 400 ml.  The water should be added while the tank is free of fuel.

Care must be taken, subsequent to this test, to ensure that the fuel tank is thoroughly dried prior to being refuelled and entering service.

Corrective Action

Correction of the problem may entail smoothing the surface of bag tanks, or ensuring the aircraft fuel system conforms to the manufacturer's requirements, or installation of a modification to incorporate improved drain points.  A number of aircraft have SBs available to install extra drain points.  Should the aircraft manufacturer not have a suitable SB available, it may be necessary to develop an approved modification for the aircraft.

If the test demonstrated that gentle rocking of the wings was necessary to move all water in the fuel tank to the drains, the aircraft owner/operator should be informed of procedures necessary to completely drain the fuel tanks of residual water.  It may also be necessary to confirm that the pilot/operator is aware of requirements in CAO 20.2, Air Service Operations Safety Precautions Before Flight, to conduct regular tests for the presence of water in the fuel.

A very simple way of draining water from your fuel system.
A very simple way of draining water from your fuel system.
Water always settles  at the lowest point

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